This invention relates to improvements in thermally actuated cut-off links, commonly referred to as thermal fuses or cut-offs, which are commonly normally closed switches of a type which respond to the ambient temperature. Typically, when the ambient temperature reaches a certain value, the cut-off link opens, interrupting the flow of electrical current therethrough. Normally closed versions of thermally actuated cut-off links, for example, are frequently physically incorporated into the windings of electric motors and to other devices requiring thermal protection and electrically connected in series with such devices so that the cut-off link will deenergize the device involved when the ambient temperature exceeds a given safety value.
Ambient thermally actuated cut-off links of the type herein considered are the subject of U.S. Pat. No. 4,246,564, issued to Olson and Borzoni, U.S. Pat. No. 4,281,309, issued to Olson, and U.S. Pat. No. 4,326,186, issued to Clay, all of which are assigned to the assignor hereof.
While the specific detailed descriptions of the above-mentioned U.S. Patents are herein incorporated by reference, an abbreviated description of the elements thereof is herewith presented to make possible a better understanding of the present invention.
Both such patents disclose cut-off links comprising a cylindrical metal casing having a first power lead passing into and insulated from the casing, the power lead terminating in a pair of integral, laterally outwardly inclining, deformable, contact-forming arms pressed against a backing member which expands the same against the inner surface of the casing. A second power lead is permanently connected, as by swaging, to the casing so that there is only one contact interface between the power leads, namely that between the arms and the casing. This contact interface is broken when the arms are bent inward by a contact arm-deforming member which is forced by spring pressure against the arms when the ambient temperature to which the link is subjected reaches a control temperature for which the link is designed. Additionally, the first power lead and contact-forming arms are preferably made of a relatively soft, very low resistance material, like silver coated copper, which, when pressed against the curved inner face of the casing, deforms somewhat to increase the contact area to minimize contact resistance.
The casing contains also a pellet of fusible material, preferably located at the initially closed end of the casing, a pair of opposed compressed spring means on opposite sides of the arm-deforming and backing members and a closure member at the initially open end of the casing. The springs are held in a compressed state by the crimping of the casing around the closure member while the springs are held compressed by external pressure applied to the closure member. Upon the melting of the pellet, the arm-deforming member is forced by one of the springs against the contact-forming arms to collapse the arms and separate them from the casing walls. Before the pellet melts, the force of the springs is not applied against the first power lead and associated contact-forming arms. Rather, these arms are held in an expanded state against said backing member by the closure member of the casing engaging the first power lead.
After the various elements described have been inserted within the initially open end of the casing and prior to closing the open end thereof with the closure member, the first power lead and associated contact-forming arms are externally pressed inwardly toward the backing member with a progressively increasing force which spreads and forces the contact-forming arms progressively more firmly against the casing walls, until a measured contact resistance between the power leads drops to a predetermined desired value (like 0.9 milliohms when measured at 1.5" between probe points on the power leads). This adjusted force will generally provide a lower contact resistance with the casing wall than is readily achievable by the force of the contact-forming arms unaided by other forces. When the contact resistance reaches this value, the power lead is anchored in its adjusted position by anchoring the closure member until it engages the first power lead.
As disclosed in U.S. Pat. No. 4,246,564, the closure is a longitudinally split compressible resilient member which initially loosely envelopes the contact arm-carrying power lead extending into the casing of the cut-off link. After the contact arms are forced against the backing member so as to produce a desired measured contact resistance, the outer edges of the initially opened casing are crimped around the split closure member to compress the same tightly against the power lead, to fix the position of the power lead in the casing and to fix the pressure of the expanded contact-forming arm against the backing member and casing walls.
As shown in this patent, the closure member is provided with a fairly substantially wide annular well surrounding the portion thereof through which the power lead passes. This well, as well as the split in the closure member, is filled with an uncured epoxy resin to seal the end of the cut-off link involved, and the epoxy is thereafter heat cured.
However, before the present invention, substantial difficulties were encountered in rapidly and effectively filling these spaces, and so the seal was ultimately made by completely enveloping the end portion of the casing and closure with a messy appearing blob of epoxy which gave the cut-off link an undesired unfinished appearance.
The power lead from which the contact-forming arms are formed passes through an arm-deforming member. The contact-forming arms incline outwardly and pass through the open end of a transverse slot or recess defined by parallel spaces in the end face of the arm-deforming member to make contact with the casing walls. When the cut-off link is blown by an overload current, and the aforementioned pellet melts, and the contact arm-deforming member moves against the contact-forming arms to collapse the same, arcs will initially develop as these contact-forming arms separate from the casing walls. Normally, these arcs are quickly quenched as the gap between the collapsing arms increases to a given distance. However, when high energy arcs are initiated by a large current flowing under the force of a high voltage, such as under the force of 18 amperes and 300 volts, there is a danger that the high energy arcs will evaporate the material of the arm-deforming member which is continguous to the point where the contact-forming arms are being separated from the casing. This evaporated material can inhibit the quenching of the arcs, with the possible result that the contact arms can be welded to the casing so the cut-off link does not open, and/or the heat of the resulting arcs can rupture the casing walls. Another aspect of the present invention overcomes this potential problem.